Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in what is called a projection exposure apparatus, which comprises an illumination device and a projection lens. The image of a mask (=reticle) illuminated by way of the illumination device is in this case projected by way of the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
Mask inspection apparatus are used for the inspection of reticles for microlithographic projection exposure apparatus.
In projection lenses or inspection lenses designed for the EUV range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, reflective optical elements are used as optical components for the imaging process.
One problem which arises in practice is that, in particular as a result of the absorption of the radiation emitted by the EUV light source, these reflective optical elements designed for operation in the EUV heat up and thus undergo an associated thermal expansion or deformation, which in turn can negatively affect the imaging properties of the optical system. This is the case in particular if illumination settings with comparatively small illumination poles are used (e.g. in dipole or quadrupole illumination settings), in which the element warming or deformation varies strongly over the optically effective surface of the reflective optical element.
Resorting to solutions for overcoming the aforementioned problem of element heating in EUV systems that are known from VUV lithography systems (with an operating wavelength of approximately 200 nm or approximately 160 nm, for example) is difficult. This is so in part because, inter alia, the number of optically effective surfaces available for active deformation compensation is relatively tightly delimited due to the comparatively smaller number of optical elements or mirrors (for avoiding light losses that are too great on account of the necessary reflections).
In order to overcome the aforementioned problem of element heating in EUV systems, the practice of using additional appliances for realizing rigid body movements and/or temperature changes in the region of the optically effective surface of the reflective optical elements designed for the operation in EUV, in particular, is known, although this increases the complexity of the systems.
Regarding the prior art, reference is made by way of example to DE 10 2010 039 930 A1.